Part of the castle needs repairs. $4$ walls need to be rebuilt, and $50$ bricks are needed for each wall. How many bricks are needed in all?
Answer: ${50}$ bricks is the same as ${5\text{ groups of ten}}$ bricks. There are $4$ walls. Total number of bricks: $\begin{aligned}&4\times{5\text{ groups of ten}}\\ =&{20\text{ groups of ten}}\\ =&{200}\end{aligned}$ ${200}$ bricks are needed.